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What is the temperature of Space?

So, my good friend and I were talking about the temperature in space. He thinks it must be hot in space, since there is no atmosphere to absorb sunlight. I disagreed. I think that the fact that there is no air in space, means there is nothing to heat up. That’s how temperature on earth works. So, surely it must be cold in space.

Can you shed light (excuse the pun) on this debate? Can you answer the question of whether it is hot or cold in space? And, therefore, can you tell us who is actually right?

Thanks, Andrew

Dear Andrew

You’re both right. Temperature is a property of matter, which indicates the average amount of kinetic energy bound up in the individual molecules (or occasionally other particles, if you want to talk about plasmas, or the surfaces of neutron stars, or other weird places) as they vibrate and shake around at the microscopic level. The higher the temperature, the more energetic the particles. The vacuum of space is, for all intents and purposes, empty of any particles, and so does not have a temperature. That’s the part that you got right. However, temperature and heat are different things. Look at the Moon, with its functional lack of atmosphere: The rocks on the day side, facing two solid weeks of unfiltered raw sunlight, heat up to hundreds of degrees Celsius, but when night falls they cool down to hundreds of degrees below zero. This is where your friend is half right: If you’re exposed to sunlight, even in space, all that heat radiation is going to warm you up. But if you’re in the shade, whether of something big like a planet or small like a satellite’s sunshield, then you’re going to cool down.
So the answer then is that space itself has no temperature, but that objects in space can have any temperature depending on how much energy is radiated towards them, how fast they’re able to radiate their internal energy away, and whether they’re generating any new energy internally (radioactive decay in the Earth’s core, or the electric heater in an astronaut’s space suit, for example).

Of course, it’s actually more complicated than that – space can actually have a temperature, although it doesn’t have much effect on things in it. The region around the Moon’s surface, for example, isn’t actually a perfect vacuum, although it certainly seems that way. The Moon, along with most other large bodies in our Solar System that don’t have atmospheres, has an exosphere which is a boundary layer of vanishingly thin gas. It might as well be a vacuum for all intents and purposes, since there are so few atoms that they almost never collide with another and so it doesn’t behave like a gas (doesn’t really exert pressure, no currents, can’t conduct heat, that sort of thing), but it is real, and it is made of particles which are in motion. And therefore it has a temperature.

There are a lot of regions like this all around the universe. Some of these regions have temperatures in the millions of degrees, but because there are so few actual particles making up the gas, the total energy is actually very low, and an astronaut passing through it wouldn’t be heated up at all. So the temperature is extremely high, but the heat is low. So basically, like so many things, our understanding of heat and temperature is based on our experiences here on Earth, where we live in a very narrow range of temperatures, pressures, speeds, and so on. This what makes up our common sense understanding of physics, which works in everyday life, but falls all to pieces when you head out into space where all those limitations go out the window.

PS: The astrophysicists reading this have no doubt been waiting for me to mention the Cosmic Microwave Background. This is an almost perfectly uniform glow shining throughout the universe from all directions, of microwave radiation. It was discovered in the middle of the 20th century by radio engineers trying to identify a mysterious noise signal in their radio receiver, and is the last fading remnant of the light and heat from the Big Bang. It is equivalent to the energy radiated by an object with a temperature of only a few degrees above absolute zero, and ensures that even in the most remote starless regions of the universe, an object will never get colder than that temperature. But that’s not strictly the temperature of space itself, so it doesn’t count as an answer for the question!

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Allen is an amateur astronomer, an IT professional, a podcaster, a father of five beautiful kids, a barely competent chess player, and the owner and publisher of the Urban Astronomer podcast and website.
He is also the director of the Citizen Science Section of the Astronomical Society of South Africa, where he tries to help ASSA members do good work with citizen science projects.